Coatings of mineral fillers bound with resinous or polymeric materials are commonly used on metal sheet structures to reduce noise, particularly where this arises from the resonant nature of the structure. These coatings are normally termed vibration damping materials and are applied to the required surfaces either as a point or a sheet by means of an adhesive. Such coatings are found also to provide a degree of sound insulation. The sound insulating efficiency of these materials is particularly related to the thermoelastic and viscous properties of the resinous or polymeric binder, but the choice of mineral filler is also important. Polymers of vinyl acetate and related copolymers are known to have exceptionally high internal damping characteristics and are, therefore, a known choice for formulating such materials.
The temperature range over which these materials are effective in providing sound insulation is a function of the softness. The softness of the materials containing vinyl acetate polymers and copolymers is readily adjusted by the use of plasticisers such as are used with nitrocellulose. An example of such a plasticiser is a low molecular weight phthalate plasticiser because they have been found generally compatible with vinyl acetate polymers and copolymers.
Since vinyl acetate polymers and copolymers are readily dissolved in polar solvents, vibration damping materials made from such polymers and copolymers are generally produced as paints for direct application or as solutions for the casting of sheet materials. Some vinyl acetate polymers and copolymers are disperable in water as emulsions which can also be used for the production of vibration damping materials in the form of paints and cast sheets.
Heretofore, however, there has been no successful production of sheets or other profiles of vibration damping materials from mineral-filled vinyl acetate polymers by thermoplastic processing of these polymers, for example by calendering.
U.S. Pat. No. 3,994,845 proposes the production of vibration damping sheets from a mineral-filled vinyl acetate polymer. A formulation of polyvinyl acetate, mica, clay, stearic acid and polyethylene glycol are mixed in a batch mixer and heated to a temperature of from 220.degree. F. (105.degree. C.) to 280.degree. F. (138.degree. C.). The resulting heated mixture is discharged in a batch and then converted into a sheet by passing it through a two roll sheeting mill whose rolls are water cooled.
This process has been found, however, not to be satisfactory for the production of such sheets. In particular, it has been found that the mixture produced by the batch mixer tends to crumble and does not sheet readily. Accordingly, the process of this specification is not suitable for the commercial production.